Intermittent black stool may be caused by physiological factors, such as eating animal liver, blood products, etc. It may be caused by pathological reasons, such as bleeding duodenal ulcer. 1. Physiological causes: eating some animal liver or blood products can cause black stools because animal hemoglobin contains a lot of iron, which combines with sulfide to produce iron sulfide, which can cause black stools. 2. Pathological reasons: when bleeding occurs in duodenal ulcer, the stool is mostly intermittent black, and the abdominal pain of the patient is often aggravated before the appearance of intermittent black stool, and the pain is reduced after the appearance of black stool. Intermittent black stools may also be seen in gastric cancer and other diseases, patients should go to specialized hospitals to clarify the cause of the disease and then under the guidance of the doctor for the next step of examination and treatment.